There's also Steel Beasts Professional (
http://esimgames.com/steel_beasts_pro.htm) that you can buy, but:
a) its expensive - 125$
b) its not really a game with campaigns, so you need to build / get missions
Other than that, its got a very good reputation as a solid, deep & detailed tank simulation.
Also see:
http://steelbeasts.com/
http://www.simhq.com/_land2/land_060a.html